1. Field of the Invention
The present invention relates to a technique of cleaning the nozzle surface of the printhead of an inkjet printing apparatus.
2. Description of the Related Art
An inkjet printing apparatus includes a head cleaning means for removing ink and dust adhered to the nozzle surface (face) of a printhead from which ink is discharged. The head cleaning means scrapes off ink and dust by pressing, against the face, a wiper made of an elastic material such as rubber, and wiping the face (see Japanese Patent Laid-Open No. 06-143597).
To implement desired cleaning performance, high accuracy is requested for wiper driving. For example, to completely remove ink and dust by one wiping, the wiper is desirably moved at a constant speed regardless of the driving load. Positions at which the wiper starts and ends contact to the face also need to be accurate so as not to generate acceleration/deceleration in a region where the wiper should be controlled at a constant speed. To achieve this, many inkjet printing apparatuses adopt a method of servo-controlling a DC motor.
When the wiper is driven by servo-controlling the DC motor, as described above, cleaning can be performed at a constant speed with respect to the driving load which fluctuates depending on the contact state to the face. However, wiper position information needs to be obtained by another means.
As a method of obtaining the wiper position information, there is a method in which a limit switch or optical sensor for detecting a wiper position is arranged, and the current position is calculated from motor rotation position information obtained from an encoder by using a detection position as the start point. As another method, a stopper is arranged at the end of a range in which the wiper moves in the cleaning operation, and when no motor rotation information is obtained from the encoder, it is determined that the wiper is positioned at the end.
Of these two methods described above, the latter method is more desirable because it does not require the cost of a sensor or the like. However, when the wiper and stopper abut against each other, an excessive force may be added, and the wiper driving mechanism and stopper require a structure resistant to the excessive force. Further, when the wiper abuts against the stopper, a flexure readily occurs, and the stop position may become unstable owing to the repulsion.
A cleaning operation in a related art will be explained with reference to FIGS. 7A to 7C and 8.
FIGS. 7A to 7C are views showing the main part of an inkjet printing apparatus in the related art, including a printhead 3 and recovery unit 5, when viewed from the side. FIG. 7A shows a standby state before cleaning, FIG. 7B shows a state during cleaning, and FIG. 7C shows a state in which cleaning has ended. FIG. 8 shows a change of the driving force during the cleaning operation in the related art.
When the cleaning operation starts in a section A in FIG. 8, wipers 7 and 8 are controlled to move at a constant speed, and contact a face 3a. In a section B, the wipers 7 and 8 wipe the face 3a at a constant speed. The driving load in the section B fluctuates depending on the state of the face 3a. For example, when a large amount of ink is adhered to the face 3a and the ink is dried, the friction coefficient and driving load increase. To maintain a constant speed, the driving force is controlled to be large, as indicated by (v). In a state in which the amount of adherent ink is small and the moisture content is high, the driving load decreases, and thus the driving force is also controlled to be small, as indicated by (vi).
When the wipers 7 and 8 pass the face 3a, the driving load temporarily decreases, as represented in a section C, and the wipers 7 and 8 abut against a stopper (not shown) and cannot move any more. In a section D, the driving force increases and reaches a predetermined upper limit value. At this time, even if the driving force is increased to the predetermined upper limit value, the wipers 7 and 8 do not move, and the motor does not rotate. From this, it is determined that the wipers have abutted against the stopper, and the energization stops.
The predetermined upper limit value is set to be larger than the load during cleaning in order to prevent the stop of the wipers 7 and 8 during cleaning owing to an insufficient driving force by setting a larger upper limit value than a maximum load on the premise of generation of the maximum load in the section B. However, every time the wipers 7 and 8 abut against the stopper, the stopper receives a shock caused by driving the wipers 7 and 8 by a driving force equivalent to the upper limit value. Therefore, the wiper driving mechanism and stopper require a structure resistant to an excessive force. Also, when the wipers abut against the stopper, a flexure readily occurs, and the stop position may become unstable owing to the repulsion.